A pink coating on the inner surface of plastic urinary tubing, which gave the impression that the urine was pink, had frequently been noted 4 to 24 hours following gastric partitioning by means of a stapler in morbidly obese patients. A study was therefore done in 187 such patients as well as in 14 patients of normal weight who had undergone abdominal surgery of comparable magnitude. Postoperatively "pink urine" was observed in 32% of the obese patients but in none of the nonobese patients; however, a pink sediment remained following centrifugation of urine collected postoperatively from all the obese patients. Microscopy of this sediment showed crystals of uric acid dihydrate; these were infrequent in the preoperative specimens but present in high concentration in the postoperative specimens, particularly those of "pink urine". X-ray diffraction analysis confirmed the nature of the crystals. Preoperatively the obese patients had high-normal serum levels of uric acid. Postoperatively in all the groups of patients the serum levels of uric acid decreased while the urine levels and the urinary clearance of uric acid increased; the last two values, however, were significantly greater, both preoperatively and postoperatively, in those who were morbidly obese. Compared with the patients who did not have "pink urine" the patients with "pink urine" were significantly more obese and had a significantly lower postoperative urine pH. The latter also had a marked postoperative increase in urine osmolality and were the only patients to have a significant postoperative decrease in urine output. Thus, the pink colour of this group's urine was attributed to precipitation of uric acid crystals, fostered by a decrease in pH and an increase in concentration of the urine.